In motor vehicles, the interaction of the internal combustion engine, elastic suspension and oscillating masses often stimulates shuddering oscillations which interfere with the behaviour of the motor vehicle. Such oscillations can also be brought about by acceleration or deceleration (overrun) operation.
Published German patent application DE 29 06 782 describes a device for damping shuddering oscillations in an internal combustion engine, whereby it is assumed that clearly measurable fluctuations in speed are associated with the shuddering oscillations. These measured fluctuations in speed are derived from the differential of an engine speed signal. The differentiated speed signal is itself then fed to the fuel quantity control means in order to counteract the shuddering oscillations.
The latter known device, which intervenes directly in the fuel quantity control, is however not suitable for all operating conditions of a motor vehicle or of an oscillated internal combustion engine, because the connection of the differentiated speed signal to the fuel quantity control means can also lead to instabilities in the control loop.
U.S. patent application Ser. No. 353,669, filed May 8,1989, now U.S. Pat. No. 4,993,389 describes a device for damping shuddering oscillations in internal combustion engines, wherein such shuddering oscillations are effectively damped, particularly during acceleration and in overrun operation, but which does not intervene directly in the fuel quantity control loop. In this latter device, the input variable to the device is the engine speed (n), the derivative (n) of the speed-dependent (n) being compared with predetermined threshold values between which the fuel quantity is arranged to be modulated in such a way that shuddering oscillations of the internal combustion engine are counteracted. The output of the device is therefore a switching signal which is used for control of its solenoid or solenoids of the fuel injection valves of the engine.
As mentioned above, the input variable to the device is the engine speed (n) and it is required that this input variable should be as free as possible from interference in order to avoid false interpretation by subsequent signal handling components of the device.
In the system disclosed in the above-mentioned U.S. patent application Ser. No. 353,669, a speed sensor is associated with the camshaft or crankshaft of the engine and provides 144 pulses per revolution of the associated shaft A divider (.div.16) reduces the number of pulses per revolution to 9. The latter pulses are processed in a circuit which deduces the rotational speed (n) of the shaft from the pulses received thereby. The resulting speed signal is then differentiated to provide the first derivative (n).
Such a device has the disadvantage that in order to remove interference it is necessary to provide a low-pass (PT1) filter between the rotational speed sensor and the differentiator.